I did the same thing early last year, rooted some supermarket watercress, planted it into very small pond plant baskets and wedged it in the crevices of the waterfall of our pond.
It took off like crazy, spread onto the surface of the water of the main pond and had to be cut back regularly. We've had so many meals from it I can't honestly remember how many, but I always make sure that I wash it thoroughly and then inspect it very closely. Sometimes I find wriggly things where the leaves join the main stem.
After more than 12 months neither I or the OH have had any ill effect so I'd say go for it!!
Belinda